1. Field of the Invention
The present invention relates to a torque converter control system having a lock-up clutch.
2. Description of the Prior Art
Generally, an automatic transmission is provided with a torque converter for amplifying an engine torque. The torque converter hydraulically amplifies the engine torque as well as transmits it. In the field of a torque converter incorporated into an automatic transmission of a vehicle, it has been proposed to provide a lock-up clutch for connecting directly an engine output member with an input member of the automatic transmission when the vehicle is in a certain vehicle operating condition.
Japanese Patent Public Disclosure (Kokai) No. 62-297567, laid open to the public on Dec. 24, 1987, discloses a control system for the torque converter having a lock-up clutch provided with a converter cover connected with an engine output shaft and a damper piston connected with a turbine shaft with the damper piston being brought into frictional engagement with the converter cover in accordance with a hydraulic pressure introduced into hydraulic chambers formed at front and back sides of the damper piston. The torque converter selectively provides a torque converter condition wherein the engine torque is hydraulically transmitted only through the torque converter, a lock-up condition wherein the engine torque is mechanically transmitted through only the lock-up clutch, and a slip condition wherein the engine torque is transmitted through both the lock-up clutch and the torque converter. Under the slip condition, the hydraulic pressure to the hydraulic chamber is controlled to accomplish a target slip rate between the converter cover and the damper piston.
This system disadvantageously necessitates various devices, such as a pressure regulator, control valve, gate valves, air bleed valves and check valves which complicate the structure thereof.
In view of the above, U.S. Pat. No. 4,989,702 discloses a simplified and compact control system of hydraulic pressure for the torque converter including a shift valve provided with a pair of juxtaposed spools taking three different positions in correspondence with the converter condition, lock-up condition, and slip condition respectively.
It should be however noted that the hydraulic control system disclosed in the U.S. Pat. No. 4,989,702 is disadvantageous in that the hydraulic pressure in the chambers of the torque converter may change abruptly and thus it is difficult to accomplish a smooth transition among the slip condition, converter condition and lock-up condition.